1. Field of the Invention
The present invention relates to a clock control system and clock control method and, more particularly, to a clock control system and clock control method having a frequency multiplication circuit and frequency division circuit for various devices which incorporate a central processing unit (to be referred to as a CPU hereinafter) and a liquid crystal display (to be referred to as an LCD hereinafter) controller serving as a peripheral functional block, and constitute a portable information processing apparatus, portable communication apparatus, and the like.
2. Description of the Prior Art
Information processing apparatuses and communication apparatuses using digital circuits have conventionally adopted various digital circuits which operate by clocks, in order to realize various functions necessary for the information processing apparatuses and communication apparatuses. Each of devices which constitute information processing apparatuses and communication apparatuses also uses a plurality of digital circuits. The clock frequency supplied to a digital circuit often changes depending on the difference between the functions of functional blocks implemented by digital circuits. To drive digital circuits, various clock frequencies with difference frequencies are used.
In each of the devices of information processing apparatuses and communication apparatuses, the frequency of a clock (to be referred to as a system clock hereinafter) which is generated as a criterion by an oscillator arranged inside or outside the device is multiplied by a frequency multiplication circuit to generate a clock having a frequency higher than the frequencies of various clocks used in the device. In distributing the generated clock to various digital circuits used in various functional blocks in the device, the clock is divided into clocks having frequencies necessary for the respective digital circuits by frequency division circuits arranged on the input stages of the respective functional blocks. The resultant clocks are supplied to the digital circuits.
The above-described technique of temporarily multiplying the frequency of a system clock by a frequency multiplication circuit in an information processing apparatus or communication apparatus, dividing the frequency of a signal output from the frequency multiplication circuit by frequency division circuits to generate clocks having frequencies necessary for respective digital circuits, and then supplying the clocks to the digital circuits of functional blocks is disclosed in, e.g., Japanese Unexamined Patent Publication No. 2002-108490 (especially see the description on pp. 1 to 3 and in FIG. 2), or Japanese Unexamined Patent Publication No. 2001-296842 (especially see the description on pp. 1 to 3 and in FIG. 1).
The clock supply circuit technique disclosed in Japanese Unexamined Patent Publication No. 2002-108490 is related to a clock supply circuit which supplies clocks to a reception circuit and a processing circuit such as a digital signal processor (DSP) in, e.g., a digital broadcasting reception LSI, and particularly to a clock supply circuit which maintains synchronization with a transmission signal by switching the frequency of a clock supplied to a reception circuit in accordance with the step-out amount of a broadcasting signal, and switches the frequency of a clock supplied to a processing circuit in accordance with the load of the processing circuit or the like.
This technique has been developed as a clock supply circuit technique capable of simplifying the circuit arrangement of a clock supply circuit and realizing low power consumption by using a low-frequency external oscillator as a system clock generation source, temporarily multiplying the frequency of a signal output from the low-frequency external oscillator by a frequency multiplication circuit, then generating clocks having frequencies necessary for various functional circuits by frequency division using separate frequency division circuits, and supplying the clocks.
The technique disclosed in Japanese Unexamined Patent Publication No. 2001-296842 is a technique related to a signal generation apparatus which generates a signal (clock) for driving a liquid crystal display panel to display a video image. This technique is related to a signal generation apparatus which can realize stable operation and arbitrarily cope with the number of building pixels of a liquid crystal display panel to be driven.
The signal generation apparatus comprises a sync detection circuit which detects a horizontal sync signal in a video signal, and a frequency division circuit which, after a system clock is multiplied by a frequency multiplication circuit, divides the frequency of the clock by using the horizontal sync signal to generate a clock having an arbitrary frequency for driving the signal electrode of a liquid crystal panel.
In many cases, information processing apparatuses and communication apparatuses comprise a frequency multiplication circuit and frequency division circuit used in the above way in order to generate, from the frequency of a system clock, various clocks having different frequencies necessary for various digital circuits in various apparatuses.
A conventional clock frequency supply method having the frequency multiplication circuit and frequency division circuit in a device which incorporates a CPU and a peripheral functional block such as an LCD controller out of various devices used in apparatuses such as a portable information processing apparatus and portable communication apparatus will be explained.
Various devices which incorporate a CPU and a peripheral functional block such as an LCD controller and are used in various apparatuses such as a portable information processing apparatus and portable communication apparatus receive power from the batteries of the portable information processing apparatus and portable communication apparatus. To keep the operation time of the apparatus long, the CPU has a low-power consumption mode function, and supply of a system clock is stopped to stop the CPU, thus suppressing power consumption of the CPU. In addition, power consumption in a circuit for generating a system clock is reduced to achieve low power consumption.
In an apparatus using a device which incorporates a CPU having the low-power consumption mode function, the low-power consumption mode of the CPU is set, and supply of a system clock is stopped to stop CPU operation, thereby suppressing power consumption. The operation time of the apparatus can be kept long since power consumption is suppressed by stopping CPU operation.
When, however, a peripheral functional block such as an LCD controller must keep operating due to its functional role even if CPU operation stops, a clock to be supplied to the peripheral functional block must be generated from a system clock by a frequency multiplication circuit and a frequency division circuit on the input stage of the peripheral functional block in order to supply a clock to the peripheral functional block.
In general, a clock frequency necessary for the peripheral functional block often suffices to be lower than a clock frequency necessary for the CPU. A signal which is output from the frequency multiplication circuit to a frequency division circuit arranged on the input stage of the peripheral functional block is identical to a signal supplied to a frequency division circuit arranged on the input stage of the CPU. A signal whose frequency is unnecessarily high is supplied to the frequency division circuit arranged on the input stage of the peripheral functional block. The signal which is temporarily multiplied to an unnecessarily high frequency must be divided to a low frequency suitable for the peripheral functional block by the subsequent frequency division circuit. As the signal is multiplied to a high frequency by the frequency multiplication circuit, wasteful power consumption in the frequency multiplication circuit and frequency division circuit increases.
In the conventional clock control system and clock control method, a clock frequency necessary for the peripheral functional block often suffices to be lower than a clock frequency necessary for the CPU. A signal which is output from the frequency multiplication circuit to a frequency division circuit arranged on the input stage of the peripheral functional block is identical to a signal supplied to a frequency division circuit arranged on the input stage of the CPU. A signal whose frequency is unnecessarily high is supplied to the frequency division circuit arranged on the input stage of the peripheral functional block. The signal which is temporarily multiplied to an unnecessarily high frequency must be divided to a low frequency suitable for the peripheral functional block by the subsequent frequency division circuit. As the signal is multiplied to a high frequency by the frequency multiplication circuit, wasteful power consumption in the frequency multiplication circuit and frequency division circuit increases.